Serum Na-Cl value from routine blood tests reflects CO2 retention in amyotrophic lateral sclerosis

BACKGROUND: Respiratory failure is the leading cause of death in amyotrophic lateral sclerosis (ALS). Early detection of CO2 retention is crucial for assessing respiratory failure severity and guiding noninvasive positive pressure ventilation (NPPV) management. However, arterial blood gas analysis is invasive and uncomfortable for routine monitoring. OBJECTIVE: We investigated whether the serum sodium-chloride difference (Na-Cl value), calculated from routine blood tests, could serve as a screening marker for CO2 retention in ALS patients. METHODS: This retrospective study included 88 ALS patients with 116 paired samples of arterial blood gas and serum electrolyte data. We analyzed correlations between Na-Cl value and blood gas parameters (HCO3-, PCO2), performed receiver operating characteristic (ROC) analysis for detecting CO2 retention (PCO2 ≥ 45 mmHg), and examined relationships with respiratory function (%FVC). RESULTS: Na-Cl value showed strong correlation with HCO3- (r = 0.78, p < 0.001) and PCO2 (r = 0.71, p < 0.001). Na-Cl ≥ 37 mEq/L demonstrated sensitivity of 85.11% and specificity of 69.57% for detecting PCO2 ≥ 45 mmHg, with negative predictive value of 87.27% (AUC = 0.842). Na-Cl ≥ 39 mEq/L achieved specificity of 92.75%. Patients with %FVC < 50% had significantly higher Na-Cl, PCO2, and HCO3- values. CONCLUSIONS: Serum Na-Cl value serves as a simple, noninvasive screening marker for CO2 retention in ALS. Na-Cl ≥ 37 mEq/L warrants blood gas analysis, while Na-Cl ≥ 39 mEq/L provides specificity >90%. This marker can be calculated from routine blood tests without additional cost, making it suitable for frequent monitoring and reducing the risk of missing intervention timing.